An active matrix substrate used in a liquid crystal display device, or the like, includes a switching element such as a thin-film transistor (Thin Film Transistor; hereinafter “TFT”) for each pixel. TFTs whose active layer is an amorphous silicon film (hereinafter, “amorphous silicon TFT”) and TFTs whose active layer is a polycrystalline silicon film (hereinafter “polycrystalline silicon TFT”) have widely been used as such switching elements.
In recent years, it has been proposed to use an oxide semiconductor, instead of an amorphous silicon or a polycrystalline silicon, as the material of the active layer of a TFT. Such a TFT is referred to as an “oxide semiconductor TFT”. An oxide semiconductor has a higher mobility than an amorphous silicon. Therefore, an oxide semiconductor TFT is capable of operating at a higher speed than an amorphous silicon TFT. Moreover, since an oxide semiconductor film is formed by a simple process as compared with a polycrystalline silicon film, it is applicable also to devices that require large areas. An active matrix substrate including an oxide semiconductor TFT (hereinafter, a “TFT substrate”) is disclosed in Patent Documents 1 and 2, for example.